Early attempts to produce physically blown foam compositions and articles from polypropylene resins melt-blended with a volatile expanding agent met with only limited success. This was due in part to the relatively low melt strength of polypropylene, which resulted in foam collapse and unacceptably high densities in the resulting products. Another factor that proved problematic with early foams was the rapid crystallization of the expanding composition, which limited the extent of expansion, and thus the thickness and minimum density, of extruded foamable compositions.
Attempts to increase the melt strength and reduce the rate of crystallization of polypropylene were successfully commercialized in a series of resins from Himont U.S.A. (now Montell U.S.A.). These resins (described as HMS or High Melt Strength resins) are readily expanded into physically blown foams.
The rigid and brittle (particularly at low temperatures) nature of polypropylene homopolymer and copolymers is a problem encountered in both foamed and unfoamed polypropylene applications. One common solution to this problem is to blend polypropylene with other resins that possess much lower bulk moduli.
Polymeric compatibility is the primary factor considered in selecting a resin for softening a crystalline thermoplastic such as polypropylene. Since the resins will necessarily be melt-blended, the various polymers must be melt-compatible, which generally requires that their individual solubility parameters be closely matched. Not only must the various polymers be mutually compatible, but the solubility of the expansion agent must be similar in each, otherwise they may segregate based upon differential solubility. Upon cooling, the polymers may not remain compatible, in which case phase-separation may occur.
Phase-separation is exploited in order to impact-modify polymeric materials, such as in the case of Acrylonitrile/Butadiene/Styrene copolymers, wherein a separate rubber phase forms during the cooling of the material, thus creating micro-domains which arrest the propagation of a fracture front. However, such phase-separating systems are rarely found in physically-expanded cellular plastics, due to the reduction of physical properties.
Known methods for reducing the modulus and/or enhancing the impact strength of polypropylene resins include the incorporation into the resin of block styrene/butadiene copolymers, such as Shell's KRATON.TM. resins, or poly(1-butene) homopolymer offered by Shell under the name DURAFLEX.TM.. Linear low-density polyethylenes are known to be at least partially compatible with polypropylene.
Most recently, with the advent of new polyolefin catalyst technology, Exxon has publically, in brochures ("EXACT Plastomers--Targeted Performance for Extrusion, Molding and Polymer Modification," Brochure #119-0594-0051-A, dated May, 1994), suggested the use of their EXACT.TM. metallocene-catalyzed linear low-density polyethylenes to improve the toughness and reduce the modulus of polypropylene homopolymer, random copolymer and impact copolymer (high-density and high-pressure low density polyethylenes are known to be incompatible with polypropylene). Absent was mention of possible uses where the blended mixtures are physically-expanded (i.e., foam applications).
During the development of the present invention, extensive evaluations were made of EXACT.TM. metallocene-catalyzed polyethylenes blended into Himont's homopolymer polypropylene (Himont #PF-814, 3.0 Melt Index, 0.900 g/cc.) on a laboratory foam extruder, using isobutane as a blowing agent. Not unexpectedly, as the level of EXACT.TM. metallocene-catalyzed polyethylene was increased, the screw torque as evidenced by the motor draw (amperage) increased considerably. In addition, the minimum density obtained was greater, and the maximum thickness was less, than homopolymer HMS.
Since metallocene-catalyzed polyethylenes are linear low density polyethylenes by design and do not shear thin like high-pressure low density polyethylenes, they possess greater melt or apparent viscosity. Higher melt viscosity leads to the detriment of processability for physically-blown foams due to shear heating. Since shear stresses are dissipated as heat, a higher apparent melt-viscosity brings about a greater rise in temperature in the resin/blowing agent blend during melt processing, such as in an extruder. Consequently, materials which do not shear-thin as LDPE result in a greater cooling demand and limited output.
Furthermore, metallocene-catalyzedpolyethylenes lack adequate melt strength to substantially expand bi-axially without collapsing so as to form closed-cell structures. Melt strength is an attribute which is best observed by measurement of extensional viscosity, and physically-blown foams are best served by materials which shear thin extentionally. Such materials exhibit a rather high apparent viscosity at low shear rates (such as during cell formation, so the cells do not collapse) but low viscosity at high shear rates (such as those typically encountered in an extruder between the barrel and the screw, so as to limit shear heating).
In light of the above mentioned shortcomings of metallocene catalyzed polyolefins, these resins were not thought to be good candidates for producing quality foamed materials, either alone or in blends with other polymers. Surprisingly, however, it has been found that excellent foams can be made by blending silane-grafted metallocene catalyzed polyethylene resins with polypropylene. The cross-linkable physically blown foams produced from such blends have numerous properties not shared by foams produced from the ungrafted blends.
One of the most notable properties achieved by the new blends is a greater propensity to shear thin. Despite the fact that the melt index of the preferred grafted EXACT.TM. resin of this invention is considerably less than either of the same EXACT.TM. resin in ungrafted form or the preferred Himont HMS, lower motor current was observed when comparing HMS polypropylene blends of grafted EXACT.TM. to the same ungrafted EXACT.TM.. A lower melt index (indicative of higher apparent viscosity at the rather low shear rates employed in the melt index test) coupled with lower motor amperage (indicative of lower apparent viscosity at the high shear rates employed in the extruder) suggests that the new grafted metallocene polyethylene blends have a greater tendency to shear-thin as compared to the non-grafted blends. Along with this enhancement in capability to shear thin came a lower density and thicker gauge, as the attached experimental results demonstrate. Other advantages were an improvement in toughness, strength and a reduction in modulus (lower compression-deflection stress), as well as the higher temperature stability inherent to polypropylene-containing blends.
In order to better highlight the differences between the present invention and the prior art, a detailed discussion of the closest related prior art follows.
Polyolefin/polypropylene blends, including polyethylene/polypropylene blends in general and LLDPE/PP blends in particular, have been generally proposed as possible choices for resins used in foam extrusion and other applications. The silane grafting of such blends has also been suggested.
Thus, U.S. Pat. No. 4,714,716 (Park) discloses a process for the production of a low density foam material having a substantially closed cell structure. Possible polymers suggested for making the material include linear olefinic polymers such as LLDPE, polypropylene, and blends thereof. Pursuant to the method, the polymeric materials are mixed with a blowing agent, which may be a volatile liquid or a solid that decomposes into gaseous materials at the extrusion temperature. A crosslinking agent, which may be a vinyl functional silane, is added to the olefinic polymer gel with the blowing agent, and serves to lightly crosslink the linear olefinic polymer with itself.
Similarly, U.S. Pat. No. 5,026,736 (Pontiff) and U.S. Pat. No. 4,702,868 (Pontiff et al.) disclose moldable polymer foam beads which are made from silane-modified polyolefins. The silane-modified polyolefin may be polyethylene, including linear low density polyethylene. The reference suggests that the polyethylene may possibly be blended with polypropylene and other compatible polymers. The blends are at least 50% by weight, and preferably 60% by weight, of the ethylene homopolymer or copolymer with the other compatible polymer. The polyolefins may be silane-grafted with vinyl trimethoxysilane and similar agents, and may be crosslinked by exposure to moisture or radiation sources.
U.S. Pat. No. 4,870,111 (Donuiff et al.) discloses the production of moldable silane-crosslinked polyolefin foam beads. The beads are produced by mixing a silane grafted polyolefin with a silanol condensation catalyst in an extruder to form a melt. A blowing agent is then injected into the melt at a rate sufficient to produce a desired foam density in the extrudate. The beads are extruded and cut, and are then exposed to moisture to induce silane crosslinking in the polyolefin foam. The polyolefin may be low density polyethylene or linear low density polyethylene. The polyethylene may be blended with up to 50% by weight of another polymer that is compatible with it. The reference suggests polypropylene as one such polymer.
U.S. Pat. No. 4,591,606 (Bergstrom) discloses a silane crosslinked polyolefin foam and a method for making the same. The foam contains a polyolefin, a chemically bound hydrolysed silane, a condensation catalyst, and a foaming agent containing water and a water carrying substance. The reference notes that possible polyolefins used in the invention include LLDPE, polypropylene, and their mixtures.
U.S. Pat. No. 5,053,446 (Salyer) discloses a composition useful in thermal energy storage. The composition may be formed from a crosslinked polyolefin having a phase change material, such as a crystalline alkyl hydrocarbon, incorporated therein. The polyolefin may be low density polyethylene or polypropylene.
U.S. Pat. No. 4,554,293 (Park) and U.S. Pat. No. 4,581,383 (Park) disclose an expandable blend of a linear olefinic polymer and a crosslinkable polymer for the production of lightly crosslinked foam compositions. The crosslinkable polymer serves to increase the melt strength of the linear olefin component, thereby allowing the use of conventional melt processing techniques for foam extrusion of the materials. The blend is about 5% to 95% by weight of a linear olefin, such as LLDPE, and from about 95% to 5% by weight of a crosslinkable polymer. The preferred crosslinking agents include organofunctional silanes. The reference notes that, without crosslinking, the foam material produced by the method is totally collapsed. Col. 7, Lines 64-65.
However, despite the general suggestions of the above noted references, the foams actually produced from polyethylene/polypropylene resins to date have been unsatisfactory. Furthermore, the advantages afforded by blends of silane-grafted LLDPE with polypropylene, particularly in foam applications, have heretofore gone unappreciated, so that these blends have not been used in practice. This is due in part to the difficulties encountered in producing satisfactory foams from the ungrafted blends and from LLDPE itself, as illustrated in the Comparative Examples set forth in the present application. As illustrated there, the ungrafted materials tend to be difficult to process, and produce unacceptably high foam densities.
A further impediment in developing foams from LLDPE/PP blends relates to the difficulties in processing LLDPE itself. These difficulties have been noted in the art. Thus, U.S. Pat. No. 5,288,762 (Park et al.) discloses a crosslinked-foam structure made from a substantially linear ethylenic polymer. The material is made by blending and heating a decomposible chemical blowing agent and an ethylenic polymer material. Crosslinking is then induced in the material, and the foamable melt material is expanded by exposing it to an elevated temperature. The resulting material is substantially linear, and has better processibility and foamability than LLDPE. The reference notes that LLDPE is difficult to process into a crosslinked foam, Col. 1, Lines 28-31, and results in a relatively high density foam structure with poor processability.
Yet another factor that has hampered the development of satisfactory LLDPE/PP foams is the high degree of unpredictability in the foam extrusion art. This is exemplified by U.S. Pat. No. 4,226,946 (Park et al.), which discloses foamed materials made from blends of low density branched polyethylene in admixture with intermediate density linear polyethylene. The reference notes that "Although a number of general principles are thought to be understood, much of the extrusion foaming technology is empirical, based on experience, and directed to very specific materials and details to produce saleable products of narrowly defined specification." Col. 1, Lines 31-36.
The use of metallocene catalysts in producing a variety of polymeric materials is known. Thus, U.S. Pat. No. 5,350,817 (Winter et al.) discloses the use of a metallocene catalysts in producing polypropylenes (see Example 1) and other polyolefins having a broad molecular weight distribution.
U.S. Pat. No. 5,278,264 (Spaleck et al.) and U.S. Pat. No. 5,329,033 (Spaleck et al.) describe the use of metallocene catalysts in making polypropylene and other polyolefins.
U.S. Pat. No. 5,186,851 (Gutierrez et al.) and U.S. Pat. No. 5,151,204 (Struglinski) describe the use of metallocene catalysts in making lubricating oil additives.
U.S. Pat. No. 5,268,115 (Gutierrez et al.), U.S. Pat. No. 5,275,747 (Gutierrez et al.), and U.S. Pat. No. 5,366,647 (Gutierrez et al.) describe the use of metallocene catalysts in making multifunctional viscosity index improver additives.
U.S. Pat. No. 5,277,833 (Song et al.), U.S. Pat. No. 5,382,698 (Song et al.), and U.S. Pat. No. 5,345,002 (Song et al.) show the use of metallocene catalysts in making dispersant additives for lubricating oils.
U.S. Pat. No. 5,391,629 (Turner et al.) discloses the use of a catalyst system having a metallocene component and an electron donor cation component in making block copolymers of ethylene and an .alpha.-olefin such as propylene. The reference notes that the block copolymers are superior to blends in that the covalent bonding between the segments eliminates interface problems, and because the block copolymers can be used as emulsifiers to strengthen blends of immiscible polymers.
U.S. Pat. No. 4,818,789 (Tomko et al.), U.S. Pat. No. 4,759,992 (Tomko et al.) and U.S. Pat. No. 4,767,814 (Bae et al.) disclose moisture curable low molecular weight polymers which have a silane grafted saturated carbon backbone. The backbone is preferably an ethylene/propylene copolymer which is prepared through the use of a metallocene catalyst.
U.S. Pat. No. 5,385,972 (Yamamoto et al.) describes a resin composition comprising an ethylene copolymer and a particulate inorganic filler. The ethylene copolymer is a copolymer of ethylene and an .alpha.-olefin, such as propylene, with a carbon number greater or equal to 3. The copolymer is formed through the use of a metallocene catalyst. The resin is used to make thin, gas permeable bodies.
U.S. Pat. No. 5,376,428 (Palazzotto et al.) describes an energy polymerizable composition containing at least one ethylenically unsaturated monomer, a polyurethane precursor, at least one epoxy monomer, a curing agent comprising an organometallic compound, and an onium salt as an oxidizing agent.
The use of silane grafting agents in grafting polyethylene and similar materials is also well known, as noted in some of the aforementioned references. Additional references include U.S. Pat. No. 4,058,583 (Glander et al.), which discloses the grafting of silane onto polyethylene. The grafting is accomplished by mixing the polymer in granulated form with a mixture of silane and peroxide. Grafting is then induced through extrusion or radiation. The grafted polymer may then be crosslinked through exposure to moisture.
U.S. Pat. No. 4,873,042 (Topcik) discloses a process for extruding a thermoplastic copolymer, whereby the copolymer is coated with an organic peroxide. Under extrusion conditions, the peroxide decomposes to provide a silanol condensation catalyst.
U.S. Pat. No. 5,047,476 (Keogh) discloses a process for crosslinking a copolymer containing a hydrolyzable silane moiety. The copolymer is mixed with dihydrocarbyltin oxide and a carboxylic acid or a carboxylate capable of forming a carboxylic acid through exposure to heat or moisture. The crosslinking is achieved by passing the mixture through a crosslinking zone where conditions are such that the carboxylic acid reacts with the dihydrocarbyltin oxide to form dihydrocarbyltin carboxylate. The crosslinking zone has a moisture content sufficient to crosslink the hydrolyzable copolymer in the presence of the dihydrocarbyltin carboxylate.
U.S. Pat. No. 4,464,425 (Voigt et al.) describes the use of a foamable, silane grafted polymer, such as polyethylene, in making shrink wrap materials.
U.S. Pat. No. 4,937,284 (Bergstrom) describes a method for manufacturing olefin/vinyl alcohol block copolymers by chemically joining polyvinyl alcohol (PVA) to a polyolefin through the agency of silane. The block copolymers obtained contain nonpolar polyolefin branches and polar polyvinyl branches.
Various blends or copolymers of polyethylene with other polymers are also known, as described above. Further examples involving non-olefinic polymers include U.S. Pat. No. 4,181,762 (Benedyk), which describes the formation of fibers from polymers having an elastic modulus of between 5,000 to 60,000 psi. The polymers are preferably copolymers of ethylene and vinyl acetate.
U.S. Pat. No. 4,725,492 (Yazaki et al.) discloses a composite heat insulating material comprising a urethane foam and a polyolefin-based resin containing carboxyl groups or a polyolefin-based resin containing hydroxyl groups.
The use of various agents to control the degree of crosslinking in foam extrusion applications is also known. Thus, U.S. Pat. No. 4,762,860 (Park) and U.S. Pat. No. 4,694,025 (Park) teach the use of alcohols to control the degree of crosslinking in a polymer prior to extrusion foaming.